The way in which tubes are laid out in a heat exchanger design is critically important if proper shell-side heat transfer is to be obtained. Research in this area of heat transfer has been conducted by many investigators over the last century. The problem of containment of the shell-side fluid, which is normally under pressure, has been most effectively resolved by the use of cylindrical shells, and the studies commonly involve internal flow-directing baffles as well as tubing layout to generate optimum heat transfer for given pressure losses.
Typical heat exchangers use tubes arrayed in a variety of tube pitches, equilateral triangles, isosceles triangles, square and rotated square pitches, and more recently a radially symmetrical pitch in which tubes are arrayed in concentric rings with an open core and an open outer annulus. These arrangements can be seen in Perry, J. H., "Chemical Engineers Handbook" and in U.S. Pat. No. 4,357,991 issued Nov. 9, 1982 naming Gordon M. Cameron as inventor (the "Cameron" patent).
To control shell-side flow, baffles are normally used in the shell and tube heat exchanger and force the fluid to cross and re-cross the heat exchanger tube bundle, generating turbulence and heat transfer in the process. The more conventional baffling arrangements include single and double segmental baffles which force the fluid to travel across the bundle in one access.
The Cameron patent describes a method of tubing layout involving concentric rings laid out so that the diagonal ligaments between tubes in adjacent rings offer the minimum cross-section for flow. This approach is very useful for tube bundles of limited transverse thickness but problems occur when a thicker tube bundle is required. Specifically, the problem occurs when the outer rings become sufficiently close to each other that the radial distances between the tubes reach a minimum and force an increase in the diagonal ligaments. Although the approach in the Cameron patent allows a second family of rings to be placed outside the main series with smaller ligaments and more tubes per ring, the discontinuity caused by the change in the ring tubing density significantly moves the outer edge of the bundle outwardly and makes the heat exhanger dimension larger. In addition there is a discontinuity in flow between the families of rings.
It is therefore an object of the invention to provide a heat exchanger having an improved tubing layout which allows compact tube packing while being capable of preserving relatively uniform fluid velocities as the fluid moves radially inwardly and outwardly through the tubes.
The invention also permits a tubing bundle to be designed which can approach closely to a square or polygonal cross-section, allowing compact large capacity units to be fabricated and shipped.
In its broadest aspect the invention provides a heat exchanger for exchanging heat between fluids and having a shell and at least first and second tube bundles extending longitudinally in said shell, each tube bundle comprising a plurality of longitudinally extending parallel tubes laid out in a set of concentric circular arcs, said tube bundles defining a central space between them, said central space also extending longitudinally in said shell and being parallel to tubes, said tube bundles having together a longitudinal axis of symmetry between them extending through said central space, the arcs of said first tube bundle having a first common center of curvature and the arcs of said second tube bundle having a second common center of curvature, said first and second centers of curvature being displaced from said axis of symmetry and from each other.
Further objects and advantages of the invention will appear from the following description, taken together with the accompanying drawings.